1. Field of the Invention
The present invention relates to an optical measuring apparatus including a multi-angle colorimeter for measuring in different illuminating or viewing directions a special effect coating such as a metallic coating and a pearl-color coating having a property that different colors are provided depending on an illuminating direction or a viewing direction, as well as an illumination system and a light detecting system for use in the optical measuring apparatus.
2. Description of the Related Art
In a metallic coating or a pearl-color coating used for a coating of an automotive vehicle or the like, flakes of aluminum or mica called bright materials are contained in a coating, which provides a metallic effect or a pearl effect. Such an effect is provided because contribution of the bright materials to reflection characteristics varies depending on an illuminating direction and a viewing direction. Optical measuring apparatus having a multi-angle geometry of the type in which illumination light is projected in a multitude of directions and reflected light is detected in one direction, or the type in which illumination light is projected in one direction and reflected light is detected in a multitude of directions are used as optical measuring apparatus for evaluating or measuring the color of the metallic coating and the pearl-color coating having the characteristic as mentioned above.
FIG. 12 is a schematic illustration showing an optical system S0 of a conventional optical measuring apparatus, in which illumination light is projected in a multitude of directions and reflected light is detected in one direction. The optical system S0 has five light sources 220, 230, 240, 250 and 260 arranged at five different angular positions as five illuminators, and a light detecting unit 300 arranged at a specified angular position as a single light detecting system. The five illuminators and the light detecting system each is constituted of a dioptric or refractive optical system. As shown by the brackets in FIG. 12, the light detecting direction of the light detecting unit 300 is set at 45 degrees with respect to a normal to an object surface 1, the illuminating directions of the light sources 220, 230, 240, 250 and 260 are respectively set at −30 degrees, −20 degrees, 0 degree, 30 degrees and 65 degrees with respect to the normal to the object surface 1, with the side where the light detecting direction is located with respect to the normal to the object surface being assumed to be positive.
The illuminating direction for causing specular or direct reflection light in the light detecting direction, namely, the specular reflection direction is set at −45 degrees with respect the normal. Accordingly, as shown in FIG. 12, the angles of the respective illuminating directions of the light sources 220, 230, 240, 250 and 260 with respect to the specular reflection direction or the anti-specular reflection angles are 15 degrees, 25 degrees, 45 degrees, 75 degrees, and 110 degrees. The optical system S0 embraces requirements on a geometry [15 degrees, 45 degrees, 110 degrees], and a geometry [25 degrees, 45 degrees, and 75 degrees], wherein the respective angles represent angles of illuminators with respect to the aspecular angles, as recommended by ASTM E2194 and DIN6175-2, 2001, which are the two primary standards for color evaluation of metallic coating and pearl coating.
An operation of an optical measuring apparatus equipped with the optical system S0 is described. First, the light sources 220, 230, 240, 250, and 260 are successively turned on by unillustrated controlling means. Light beams emanated from the respective light sources 220, 230, 240, 250, and 260 are collimated into parallel beams by collimator lenses 122, 123, 124, 125, and 126, and the object surface 1 is illuminated by the illumination beams from the respective illuminating directions. The beams reflected by the object surface 1 at 45 degrees with respect to the normal (45 degree anormal), namely, object beams are converged on an object slit 350a in a slit plate 350 through a light receiving lens 330 of the light detecting unit 300. The converged object beams are incident onto a diffraction grating 370 as parallel beams after passing through a focusing lens 360 for dispersive reflection with respect to each wavelength component. Thereafter, the dispersed and reflected beams are converged by the focusing lens 360, and incident onto an object array 380a in a sensor array 380 shown in FIG. 13A, with a dispersed image of the object slit 350a shown in FIG. 13B being formed thereon.
Also, parts of output beams from the light sources 220, 230, 240, 250, and 260 are guided to incident ends of monitoring optical fibers 220f, 230f, 240f, 250f, and 260f as reference beams to monitor fluctuation of illumination beams emitted from the five illuminators 220, 230, 240, 250, and 260. Exit ends of the monitoring fibers 220f, 230f, 240f, 250f, and 260f are arrayed on a reference slit 350b in a slit plate 350 shown in FIG. 13B. Similarly to the object beams, the reference beams successively emitted from the exit ends of the monitoring fibers 220f, 230f, 240f, 250f, and 260f are incident onto a reference array 380b in the sensor array 380, with a dispersed image of the reference slit 350b being formed thereon.
Signals indicative of spectral intensities of the object beams and the reference beams that have been incident onto the object array 380 are processed by an unillustrated processing circuit as spectral intensity data, which is sent to unillustrated controlling/computing means. The controlling/computing means calculates spectral reflection coefficients of the object surface in the respective reflection directions based on the spectral intensity data of the object beams and the reference beams by using illumination beams in the respective illuminating directions, and converts the spectral reflection coefficients into a color value or the like, according to needs.
In the aforementioned optical system S0, it is necessary to radially arrange the illuminators each having a long axial length from the corresponding light source to the object surface. Also, it is necessary to set the length from the object surface to the respective collimator lenses sufficiently long to avoid interference between the adjoining collimator lenses arranged in two different angular positions. The following is an example of the latter drawback. Let it be assumed that in the arrangement shown in FIG. 12, the illumination area on the object surface 1 has 15 mm in size. Then, the distance from the object surface 1 to the collimator lens 122 (123) must be 80 mm or more to avoid interference of the collimator lenses 122 and 123 in the space between the two illuminators having the angles of 15 degrees and 25 degrees with respect to the specular reflection direction (15 degree aspecular and 25 degree aspecular). In an actual arrangement, since lens barrels for holding the respective collimator lenses therein are provided in the optical measuring apparatus, it is necessary to set the distance between the object surface and the respective collimator lenses to such a value that the adjoining lens barrels do not interfere with each other. Consequently, increase in the size of the optical measuring apparatus is unavoidable. If such a large-sized optical measuring apparatus is made portable, an operator may feel difficulty in using the optical measuring apparatus in an actual measurement field.